Star Testing Your Telescope
Using the Airy Disk and Rings to determine optical excellence
by: Clay Sherrod
This was in response to an inquiry about star testing and folks concerned about everything being "perfect".
Star testing a telescope is easy, but should never be done under poor conditions. It will lead to unnecessary worry unless done under perfectly steady skies.
Sometimes when things are going so well, we tend to "look for" problems that we think "should" exist....the old "too good to be true," nature of human-folk.
Let's talk about the star test for a second, though. First, in focus, there is probably a condition that may arise with "too many diffraction rings" from an Airy pattern around a bright star, but I would like that luxury. With my telescopes on a very clear night and, say Arcturus or Vega, I can see up to seven rings plus the central star "disk." Smaller telescopes reveals as many as five on a good night. Diffraction rings are good....if you were NOT getting them, or if they met/combined/mingled then you would have a problem. But you should never expect to see Airy rings around stars, even on better than average nights.
The Airy pattern is one of the most stringent of all star tests.
Now let's talk about your out-fo-focus star disk test. Intra-focus and extra focus images on a very clear night (always choose a bright star nearly overhead on a very steady night) will reverse themselves in nearly all catadioptic/compound telescopes as you move in and out of focus. It is common for one to appear "sharper" than the other. Use about 150x per inch for the test. If the out-of-focus star image (a disk) appears to wiggle or oscillate, forget the test,....the seeing is too poor.
Also look for optical astigmatism while moving inside and outside of focus; when the star is very slightly inside of focus and shows an ELLIPTICAL rather than circular pattern, and then as you move to OUTSIDE of focus the same elliptical disk appears, except rotated 90 degrees, then you have astigmatism with something in your optical system: main optics, eyepiece, etc.
With a bright star at high magnification in your scope you should see very, very fine and close concentric rings within the out-of-focus star image, with the donut hole of the secondary right in the center of the disk; an offset secondary shadow indicates a collimation problem....OR it might indicate that your scope is simply NOT CENTERED on the star. You will note that the "hole" appears to shift to one side as you move the star closer to the edge of the field.
The tiny little rings that you should see should be essentially un-disrupted from being cleanly separated from the others; if they appear to merge periodically, that is okay. However, if you see major distortions on the disk, like a splash in a puddle or such, it is an indication of some degree of optical flaw. I doubt you will see this.
Always make sure your telescope has "equalized" to the outside air for AT LEAST two hours before doing any star test. If your telescope is a different temperature than the air, you will not ever get good results.
Important note: small telescopes have a distinct advantage (see my GUIDE on this ASO site regarding seeing and transparency) over large ones in average-to-poor seeing. In short, the greater the aperture the greater the resolving power; this means the larger scope can resolve AIR TURBULENCE even better as well, not just a festoon on Jupiter, or a faint star. This is why small scopes show considerably more detail on nights of poor seeing than large ones at the same star party.
There is a very good reason for this: the air is boiling with tiny vortexes call EDDY CURRENTS. These tiny swirling whirlwinds are typically about 0.43 arc seconds across. A 10-inch telescope of good quality has a resolving power of 0.46 arc seconds, just higher than necessary to resolve the air turbulence. A 12 inch telescope might resolve them clearly and that very added aperture is enough to render better images in the 10-inch than in the larger aperture. In the small the scope, the LARGER is the Airy disk (the central image of the in-focus bright star), appearing more like an actual planetary disk than with larger scopes. This, obviously is simply a result of the greater resolving potential of larger scopes.
You've fi you've a good scope....hide it. There are other enthusiasts out there who would die for it!
Good luck and continued excellence in your observing!
P. Clay Sherrod